Render settings for rendering an animation (with fire/smoke simulation)
Rendering 4k means almost x4 the time of a 1080p render, and adding a second card with the same power won't double the performance. However, rendering with two instances of blender, on two separate cards and on different frames, and you will have the best performance you can get from a single machine.

Also the OS counts, my experiments shows that on linux it's faster.

Some Advices:


Turn off caustic, and use some clamp to greatly reduce render time and be conservative with light bouncing setting.
Use all the rendering pass on one frame to discover the most noisy one, and then use branched path tracing to cast more ray where is really needed.
Bake everything you can bake in advance.
I bet you can reduce the time a lot if you follow these advices.
Render out on a series of png/exr, don't output to a video format directly.


Edit with more info:

For basic settings for light paths you can select "limited global illumination" in the dropdown (this will also uncheck caustics). 


Filter glossy it's also sometimes useful.
Clamp settings. It's under Sampling (still in the render tab) start setting Clamp indirect to 1 or higher (it works the opposite lower values more clamping but I don't advice to go lower then 1).


A tutorial on "branched path tracing".

And again, pre-baking a combined pass and then rendering it using an emission shader will save time for every frame a lot!

These are general advices and don't apply specifically to fire/smoke because every render is different depending on the material used. 

Of course if you need to make a long sequence, and you reach the limit of optimization you can always use a render farm. However, you can go a long way by just preparing the scene before rendering.